You can't get to maximum ram without things being automatically shut down or cached by Android.
There are 6 different running app categories, each with a marker that says, if free ram gets below this mark then start caching or shutting down apps and services in this category.
The closer you get to maximum ram usage, the slower your phone will likely perform, while Android steps in and does memory management.
No Android phone ever starts up with all of its ram free. If you had a phone where you thought so, there was something wrong with the ram reports.
Neither can you configure one to have little or no ram left at startup.
This isn't a device with a swap system. Apps are running in memory or they're not.
Good luck in your quest to use more memory at startup. Still not sure why you want that, but that's your business.
PS - 4G has nothing to do with it.
And a lot of task managers simply lie about how much ram is in use and by what.